- Email: [email protected]
Kinetics of Mild Steel Corrosion in Aqueous Acetic Acid Solutions
J. Mater. Sci. Technol., 2010, 26(3), 264-269.
Kinetics of Mild Steel Corrosion in Aqueous Acetic Acid Solutions S.K. Singh1,2)† and A.K. Mukherjee1) 1) Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India 2) Department of Chemistry, University of Delhi, Delhi 110007, India [Manuscript received February 16, 2009, in revised form October 20, 2009]
The kinetics of mild steel corrosion in aqueous acetic acid solution has been investigated by weight loss and polarization techniques at 25, 35 and 45◦ C. The weight loss of mild steel at room temperature (25◦ C) has been found to be quite significant, indicating poor corrosion resistance in acetic acid. The maximum corrosion rate has been observed in 25% acetic acid solution at all three experimental temperatures. The decrease in corrosion rate after attaining a maximum value has been attributed to the deposition of corrosion product on the surface. Anodic polarization curves exhibit active behaviour at each concentration and temperature with a shift towards higher current density region and increased corrosion rates at higher temperatures. The cathodic polarization curves are almost identical irrespective either of the concentration of acetic acid or temperature. The results obtained by both the techniques are in good agreement, while the surface studies support the conclusions drawn from the weight loss method. KEY WORDS: Corrosion; Mild steel; Acetic acid
1. Introduction It is well known that mild steel is one of the best preferred materials for industry due to its easy availability and excellent physical properties, but its use is restricted in acidic environments because of the susceptibility towards corrosion. It is cheaper than wrought iron and stronger and more workable than cast iron. On the other hand, from a commercial point of view, acetic acid is by far the most important organic acid among the lower carbon acids in the aliphatic series. It is applied in the manufacture of drugs and pharmaceuticals, rayon and plastics, rubber and silk industries. It is reasonable to expect mild steel to be used as reaction vessels or storage tanks by industries where the acetic acid is manufactured or used as a reactant. The effect of acetic acid on the kinetics of mild steel corrosion is a complex and serious problem and requires thorough investigation. It may be worthwhile to examine how carboxylic acids and their † Corresponding author. Ph.D.; Tel.: +91 98 68625466; E-mail address: sk− [email protected] (S.K. Singh).
derivatives are often used in conjunction with the organic corrosion inhibitors to enhance the efficiency of corrosion prevention and control practices[1] . In case of acetic acid, the acetate helps to passivate the corroding mild steel electrode. Aliphatic acids are adsorbed through their carboxylic groups and the inhibition is independent of the chain length[2] . Amino acid adsorption occurs through both protonated–COOH and NH2 groups[3] . Most organic inhibitors adsorb on the metal surface by displacing water molecules on the surface, forming a compact film[4–6] . It has been observed that short chain length carboxylates are not quite effective in inhibition of mild steel corrosion in neutral aqueous solution because they fail to block completely the surface sites for anodic dissolution[7] . A mixture of carboxylic acid and tertiary amine has been found to provide better inhibition for steel than either of the inhibitors taken individually[8] . Nitrogen is considered as the active centre of the amine. It is able to exchange electrons liberated by the metal during the first step of anodic dissolution. The physisorbed species react with the initial corrosion products to form a complex precipitate on the steel sur-
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269 [9,10]
face, which decreases the corrosion rate . Increase in the thickness of the layer is expected due to the synergistic effect of carboxylic acid and tertiary amine[10,11] . The behaviour of iron in neutral and weakly acidic solutions has been treated in relatively few publications. The corrosion behaviour of AISI 304L and 316L stainless steels, by Otero et al.[12] and 430 stainless steel in formic-acetic acid solutions have been studied by Sekine et al[13,14] . Abdel Aal et al.[15,16] studied the anodic behaviour of mild steel in deaerated carboxylic acid solutions and corrosion behaviour of steel in acetic, oxalic and citric acids. The influence of water content on corrosion of metals in monocarboxylic acids has been observed by Constantinescu and Heitz[17] . Performance of duplex stainless steels in acetic acid has been studied by Kangas and Newman[18] and a study on corrosion by lower aliphatic organic acids has been made by Teeple[19] for ferrous and non-ferrous alloys. The usual influence of hydrogen ions has been shown to apply to the dissolution process of iron in the solution of sodium salt of acetic acid by Nord and Bech-Nielsen[20] . Common corrosion problems encountered by various construction materials in acetic acid have been discussed in a report[21] . The effect of acetic acid on the corrosion of carbon steel for 500×10−6 –5000×10−6 concentration has been reported recently by Okafor and Nesic[22] . Corrosion behavior of 316L stainless steel in acetic acid solution over the concentration range 70%–90% has been studied by Turnbull et al[23] . Recently the corrosion behaviour of duplex stainless steels in organic acid aqueous solutions has been reported by Invernizzi et al.[24] and the effect of organic compounds on the electrochemical behaviour of steel in acidic media has been reviewed by Maksoud[25] . We have published the results of electrochemical behaviour of mild steel in glacial acetic acid with and without formic acid[26] . Marked dependence of corrosion behaviour on the concentration of the acetic acid and experimental temperature has been observed for AISI 316 stainless steel by Sekine et al.[27] and for mild steel by Singh and Gupta[28] . However, the investigation carried out by Singh and Gupta was for shorter exposure time and at selected small concentrations, and also did not report surface studies. In short, the kinetics of mild steel corrosion over the whole range of aqueous acetic acid solutions has not been represented adequately in literature. The aim of the present study was to investigate the corrosion behaviour of mild steel over a wide range of concentration and test immersion periods. The weight loss results have also been supplemented by surface study investigations. 2. Experimental In our experiments, analytical reagent (AR) grade glacial acetic acid and mild steel samples having composition of C, 0.23; Mn, 0.11; Si, 0.02; P, 0.02; S, 0.02;
265
Ni, 0.02; Cu, 0.01; Cr, 0.01; and remainder Fe (wt pct) were used. For weight loss experiments, coupons were cut in the form 3 cm×3 cm×0.1 cm. The surface of the specimen was polished using successive grades of emery paper, from 1/0 to 4/0, washed with soap solution and then by running tap water followed by double distilled water, finally degreased with acetone, dried in air and kept in a desiccator. Later, the samples were weighed and immersed in 300 mL of test solution in a glass beaker for a fixed interval of time. The samples were kept in the beaker in such a way that both the surfaces were in contact with the solution. After removing the specimens from the electrolyte, they were washed thoroughly, dried and reweighed. The surface morphology of the samples before and after performing the weight loss experiments was observed carefully and analyzed by a JEOL JSM-840A scanning electron microscope (Japan) at an operating voltage of 10 kV. For polarization measurements, the working electrodes were cut in rectangular form 3 cm×1 cm×0.1 cm. The exposed area was only 1 cm×1 cm and considering both sides the total exposed area was 2 cm2 . Remaining portion of the sample was covered with wax to avoid any possibility of electrolytic contact. A three-necked glass cell assembly with a 1 cm×1 cm counter electrode and a saturated calomel electrode (SCE) with a luggin probe containing saturated solution of KNO3 as a reference electrode was used. The open circuit potential (OCP) was recorded after it attained a constant value. The measurements were made at 25, 35 and 45◦ C and potentials were quoted with respect to the SCE. Potentiostatic polarization was achieved using a Wenking POS 73 potentiostat (Bank Elektronik, Germany), manually changing the potential stepwise at a rate of 10 mV/min. The temperature was kept constant with the help of an air thermostat with the variation of ±0.2◦ C in the experimental temperature. 3. Results and Discussion 3.1 Corrosion behaviour by weight loss method The corrosion rate of mild steel in different concentrations of acetic acid for exposure time 24 h and 168 h has been determined by weight loss method at 25, 35 and 45◦ C. The corresponding curves for specific conductivity and corrosion rate of mild steel as a function of concentration have been plotted in Fig. 1(a) and (b). It is observed that there is a rapid increase in the corrosion rate with increase in the concentration of acetic acid till it reaches a maximum at 25%. Any further increase in the concentration of acetic acid results in an exponential decrease in the corrosion rate of mild steel at each temperature. Similar results were found by Sekine et al.[14] for the stainless steel-acetic acid system. The marked decrease in corrosion rate at higher concentrations of acetic acid is assumed to be a consequence of the increase in the viscosity of
266
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
Fig. 2 Variation of corrosion rate of mild steel in 20% acetic acid with exposure time at different temperatures
Fig. 1 Variation of corrosion rate and conductance with concentration at different temperatures
the solutions due to the formation of a dimer, resulting in a decrease in the mobility of the ions. Acetic acid is generally associated to form a dimer and polymer at higher concentrations[29] . Besides this, an increased concentration leads to an increase in electrostatic ion-ion interactions and decrease in the degree of acetic acid dissociation. Maximum value of corrosion rate of SS41[29] in 30% and of copper[30] in 20% acetic acid have been reported. The corrosion behaviour observed for 168 h exposure is nearly the same as for 24 h. From the nature of curves in Fig. 1(a) and (b), it is evident that as the temperature of the system is increased, the corrosion rates of mild steel increase at all the concentrations of acetic acid, irrespective of the time of exposure. The dependence of corrosion behaviour of mild steel on acid concentration and temperature in the present case may also be explained in terms of the change in specific conductivity. The concentration of acetic acid for the maximum corrosion rate of mild steel almost coincides with the maximum in specific conductance. The increase in corrosion rate with temperature may likewise, be attributed to the increase in the conductivity of acetic acid solution as the temperature is increased. The corrosion rates of mild steel in 20% acetic acid solution have been determined by the weight loss method for 6, 12, 24, 48, 72, 96, 120, 144 and 168 h exposure time at 25, 35 and 45◦ C. The curves representing these values are shown in Fig. 2. The variation of corrosion rate of mild steel with exposure time at
20% acetic acid is found to be temperature dependence. It can be seen that initially with increasing time of exposure from 6 to 24 h, the corrosion rate of mild steel increases irrespective of temperature. However, on further increase of exposure time, the corrosion rates at 35 and 45◦ C exhibit different variation than that at 25◦ C. The variation in corrosion rate with the time of exposure between 6 and 24 h may be due to the increase in conductance of the solution as a result of continuous addition of Fe2+ caused by the corrosion of mild steel. The extent to which corrosion is promoted depends on the concentration of Fe2+ ions[31] . The decrease in corrosion rate after 24 h may be either due to a decrease in solubility of the electrolyte as it slowly becomes saturated with the corrosion product or the formation of some surface film which retards the corrosion of mild steel. The formation of a surface film is unlikely, as evident from the significant dissolution of the samples and also from the polarization data mentioned in the following section. The decrease in corrosion rate must have occurred due to precipitation of the corrosion product. This hypothesis is supported by the fact that at higher temperatures the corrosion rate starts increasing at a lower immersion period. Due to the enhanced rate at higher temperatures, the saturation limit would have been attained earlier. This behaviour is further supported by the results of surface study and has been discussed at 25◦ C as representative. As corrosion progresses in acetic acid solution, mild steel gradually starts dissolving in the acid. After 6 h of the immersion, the sample loses its metallic lustre and the polished streak is not seen on the surface. A similar phenomenon is observed after 12 h with more unevenness on the surface. After 24 h of exposure, the test sample is seen to be uniformly covered with a very thin non-sticky layer of dark gray corrosion product. On further increasing the immersion period, the amount of deposited material on the surface increases. Thus, the increase in corrosion rate
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
267
Fig. 3 Scanning electron microphotographs of mild steel specimens exposed to acetic acid solution (after weight loss experiment): (a) uncorroded polished samples, (b) 20% acetic acid for immersion period 24 h, (c) 20% acetic acid for immersion period 96 h, (d) 20% acetic acid for immersion period 168 h
observed by weight loss tests from 6 to 24 h is usually due to the increase of the exposed area because the surface becomes rough, leading to enhancement of the total surface area. Moreover, the number of Fe2+ ions increases in the solution due to continuous dissolution of material. Further, as the time of exposure increases beyond 24 h, the overall weight loss increases with time but the net corrosion rate shows a decreasing tendency up to 144 h. This decrease is initially fast but slows down with increasing time of exposure. This can be explained by the fact that the material deposited on the surface acts as a barrier leaving less active area on the surface. Corrosion process does not stop completely but progresses at a slow rate suggesting that there is formation of a nonadherent and non-compact film, which is unable to protect the material completely from further corrosion but only retards the process up to some extent. After a certain amount of deposition, protection is very slightly affected as it becomes controlled almost exclusively by porosity. It appears that after 120 h, the porosity attains a value that is practically unaffected by the amount of corrosion product presented on the surface. On further increasing exposure time after 144 h, the corrosion rate starts increasing again. It may be assumed that the deposited layer starts breaking beyond 144 h due to its porous and bulky nature. The surface morphology of the specimens was analyzed by scanning electron microscopy (SEM) after removing the corrosion product. The microphotographs are given in Fig. 3(a)–(d). Figure 3(a) represents the uncorroded, polished sample. The entire surface is smooth and homogeneous. Figure 3(b) shows the corroded sample exposed for 24 h, which corresponds to
the maximum corrosion rate. Figure 3(c) represents the corroded sample exposed for 96 h. This is the intermediate value of exposure time at which a decreasing tendency in corrosion rate is observed. Figure 3(d) indicates the corroded sample after 168 h of exposure, which is the maximum time of exposure. The SEM investigation results indicate a uniform attack by acetic acid. SEM analysis shows a very limited attack after 24 h (Fig. 3(b)). The corroded portion is much smaller than the total surface. With increase of the immersion period, the extent of corrosion is increased, which can also be seen from weight loss tests. After 96 h of the test, the surface appears uncorroded in several zones (Fig. 3(c)). This behaviour could be explained by assuming a layer of corrosion product acting as a barrier against liquid metal attack. The corrosion attack becomes more prominent and vigorous after 168 h (Fig. 3(d)) due to larger availability of the exposed area for attack. 3.2 Electrochemical behaviour of mild steel in acetic acid solution The potentiostatic anodic and cathodic polarization behaviour of mild steel in different concentrations of acetic acid, i.e. 5, 10, 20, 25, 30, 35, 40, 60, 80 and 90 percent has been studied at 25, 35 and 45◦ C, respectively. Figure 4 illustrates this behaviour of mild steel in different concentrations of acetic acid at 25◦ C. The figure shows active corrosion behavior of mild steel under all experimental conditions. Active nature of anodic curves for mild steel in 20% acetic acid solution has been shown by other investigators in the presence and absence of inhibitors[32] . Well-defined Tafel regions, followed by distinct limiting current densities are observed at all concentrations of the acid at all
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
268
Table 1 Electrochemical parameters of mild steel in different concentrations of acetic acid at 25, 35 and 45◦ C Concentration of acetic acid/% At 25◦ C 5 10 20 25 30 35 40 60 80 90 At 35◦ C 5 10 20 25 30 35 40 60 80 90 At 45◦ C 5 10 20 25 30 35 40 60 80 90
icorr /(μA/cm2 )
Ecorr /mV
iL /(mA/cm2 )
Anodic Tafel slope (βa )/(mV/decade)
Cathodic Tafel slope (βc )/(mV/decade)
Corrosion rate /mpy
72.44 95.50 151.36 151.36 125.89 120.23 104.71 66.07 19.95 3.55
−630 −667 −597 −620 −600 −600 −600 −545 −490 −435
5.01 6.67 7.58 7.24 6.02 4.17 3.31 1.58 0.25 0.036
160 172 188 180 192 192 204 200 214 285
200 204 227 220 226 208 250 219 196 250
33.15 43.70 69.26 69.26 57.61 55.01 47.92 30.23 8.33 1.62
112.20 173.78 260.02 260.02 199.53 181.97 165.96 107.15 36.31 10.0
−610 −655 −600 −615 −575 −580 −585 −575 −480 −460
5.49 7.94 13.18 11.79 7.58 4.36 3.63 2.19 0.46 0.095
150 185 217 250 192 192 200 208 250 250
238 155 273 250 250 250 250 211 289 389
51.34 79.52 118.98 118.98 91.30 83.27 75.94 49.03 16.61 4.58
190.55 257.04 389.19 389.19 354.81 346.74 275.42 144.54 52.48 14.12
−636 −625 −625 −600 −595 −600 −575 −535 −490 −410
6.61 8.71 15.84 12.58 10.47 5.25 4.36 2.63 1.99 0.46
166 200 230 293 250 278 285 250 275 239
500 294 278 429 286 375 321 346 500 425
87.19 117.62 178.09 178.09 162.36 158.67 126.03 66.14 24.02 6.46
Fig. 4 Anodic and cathodic polarization curves of mild steel in different concentrations of acetic acid at 25◦ C
the temperatures. The potential range over which the Tafel law followed does not depend much on the concentration of the electrolyte. This result is contrary to that obtained by Singh and Gupta[28] , whose observation for mild steel is not active but passive and transpassive behaviour as well. The values of corrosion current density (icorr ), corrosion potential (Ecorr ), limiting current density (iL ), the Tafel slope of anodic and cathodic curves (βa ) and (βc ) and corrosion rate calculated from the polariza-
tion curves at different concentrations of acetic acid at 25, 35 and 45◦ C, respectively, have been recorded in Table 1. By comparing these data with those of weight loss method, it may be inferred that the corrosion behaviour observed by these two methods is similar. However, at higher concentrations, viz. 80% and 90%, discrepancy between these two methods is observed. At the concentration of 90%, the electrochemical data exhibits tremendous decrease in the corrosion rate as compared with that obtained by weight loss method. This occurs due to the very high resistance of the solution at higher concentrations. Since acetic acid is a weak acid, it is feebly dissociated at higher concentrations. Thus, decrease in dissociation and increase in viscosity reduce its conductance. From Fig. 4, an initial regular shift in the cathodic polarization curves towards higher current density with increase in the concentration of the acid is observed. The curves become steeper as the concentration is increased. When the concentration is increased from 5% to 20%, the shift towards higher current density region is maintained over the whole range of potential. In the range from 20% to 40% acid concentration, the curves either overlap at some regions or even cross one another at others without showing any regular pattern.
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
269
REFERENCES
Fig. 5 Plot of logicorr vs 1/T for various concentration of acetic acid
On the basis of above observations, the cathodic polarization behaviour of mild steel can be considered to be a hydrogen evolution reaction. With increase in the acid concentration, the concentration of undissociated acetic acid molecules increases, thereby decreasing free hydrogen ion concentration. The increase in cathodic reaction rate may be explained by the increase in the molecular concentration of acetic acid. The increase in corrosion rate with temperature is due to an increase in the extent of adsorption, assuming that the acetic acid is chemisorbed on the surface of the cathode. Ultimately, this leads to an increase in the rate of hydrogen evolution reaction. The plot of logicorr derived from the extrapolation of anodic and cathodic polarization curves at different concentrations of acetic acid against 1/T (T -absolute temperature), have been shown in Fig. 5. A linear relationship is observed indicating that these parameters are under activation control. 4. Conclusions (1) The corrosion behaviour of mild steel has a marked dependence on concentration, temperature and exposure time. A correlation between the corrosion rate and the state of the mild steel surface is observed for various immersion periods. 20%–30% acetic acid is more corrosive towards mild steel. (2) The anodic polarization curves of mild steel in acetic acid solution show active behaviour over the whole range of applied potential at each concentration and temperature. At higher concentration of acid (90%), the anodic current decreases due to very large internal resistance offered by the solution. (3) The cathodic reaction results in the evolution of hydrogen. The values of logicorr increase linearly with the decrease of 1/T . Acknowledgement The authors would like to gratefully acknowledge the many helpful suggestions provided by Prof. M.M. Singh, Department of Applied Chemistry, I.T., B.H.U., Vatanasi (India) in his discussion with the authors.
[1 ] R.D. Marco, Z.T. Jiang, D. John, M. Sercombe and B. Kinsella: Electrochim. Acta, 2007, 52, 3746. [2 ] M.N. Moussa, M.M. El-Tagoury, A.A. Radi and S.M. Hassan: Anti-Corros. Methods Mater., 1990, 37, 4. [3 ] R.M. Saleh and A.M. Shamseld: Corrosion Sci., 1972, 12, 689. [4 ] M. Bouklah, A. Ouassini, B. Hammouti and A. El Idrissi: Appl. Surf. Sci., 2006, 252, 2178. [5 ] S. Muralidharan, K.L.N. Phani, S. Pitchumani, S. Ravichandran and S.V.K. Dyer: J. Electrochem. Soc., 1995, 142, 1478. [6 ] P. Kern and D. Landolt: Electrochim. Acta, 2001, 47, 589. [7 ] U. Rammelt, S. Kohler and G. Reinhard: Electrochim. Acta, 2008, 53, 6968. [8 ] P. Bommersbach, C. Alemany-Dumont, J.P. Millet and B. Normand: Electrochim. Acta, 2005, 51, 1076. [9 ] N. Tsuji and K. Nozawa: Corrosion Sci., 2000, 42, 1523. [10] T. Suzuki, H. Nishihara and K. Aramaki: Corrosion Sci., 1996, 38, 1223. [11] M. Duprat, F. Dabosi, F. Moran and S. Rocher: Corrosion, 1981, 37, 262. [12] E. Otero, A. Pardo, M.V. Utrilla, F.J. Perez and C. Merino: Corrosion Sci., 1997, 39, 453. [13] I. Sekine, S. Hatakeyama and Y. Nakazawa: Corrosion Sci., 1987, 27, 275. [14] I. Sekine, S. Hatakeyama and Y. Nakazawa: Electrochim. Acta, 1987, 32, 915. [15] M.S.Abdel Aal and M.H. Wahdan: Brit. Corros. J., 1981, 16, 205. [16] M.S.Abdel Aal, M.H. Wahdan and G.K. Gomma: Mater. Chem. Phys., 1995, 39, 290. [17] E. Constantinescu and E. Heitz: Corrosion Sci., 1976, 16, 857. [18] P. Kangas and M. Newman: Anti-Corros. Methods Mater., 1998, 45, 233. [19] H.O. Teeple: Corrosion, 1952, 8, 14. [20] H. Nord and G. Bech-Nielsen: Electrochim. Acta, 1971, 16, 849. [21] H.O. Teeple: Corrosion, 1957, 13, 79. [22] P.C. Okafor and S. Nesic: Chem. Eng. Commun., 2007, 194, 141. [23] A. Turnbull, M. Ryan, A. Willetts and S. Zhou: Corrosion Sci., 2003, 45, 1051. [24] A.J. Invernizzi, E. Sivieri and S.P. Trasatti: Mater. Sci. Eng. A, 2008, 485, 234. [25] S.A. Abd El Maksoud: Int. J. Electrochem. Sci., 2008, 3, 528. [26] S.K. Singh, A.K. Mukherjee and M.M. Singh: IJCT, 2008, 15, 68. [27] I. Sekine, A. Masuko and K. Senoo: Corrosion, 1987, 43, 553. [28] M.M. Singh and A. Gupta: Corrosion, 2000, 56, 371. [29] I. Sekine and K. Senoo: Corrosion Sci., 1984, 24, 439. [30] V.B. Singh and R.N. Singh: Corrosion Sci., 1995, 37, 1399. [31] O. Krim, A. Elidrissi, B. Hammouti, A. Ouslim and M. Benkaddour: Chem. Eng. Commun., 2009, 196, 1536. [32] M.A. Quraishi and H.K. Sharma: J. Appl. Electrochem., 2005, 35, 83.
Kinetics of Mild Steel Corrosion in Aqueous Acetic Acid Solutions S.K. Singh1,2)† and A.K. Mukherjee1) 1) Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India 2) Department of Chemistry, University of Delhi, Delhi 110007, India [Manuscript received February 16, 2009, in revised form October 20, 2009]
The kinetics of mild steel corrosion in aqueous acetic acid solution has been investigated by weight loss and polarization techniques at 25, 35 and 45◦ C. The weight loss of mild steel at room temperature (25◦ C) has been found to be quite significant, indicating poor corrosion resistance in acetic acid. The maximum corrosion rate has been observed in 25% acetic acid solution at all three experimental temperatures. The decrease in corrosion rate after attaining a maximum value has been attributed to the deposition of corrosion product on the surface. Anodic polarization curves exhibit active behaviour at each concentration and temperature with a shift towards higher current density region and increased corrosion rates at higher temperatures. The cathodic polarization curves are almost identical irrespective either of the concentration of acetic acid or temperature. The results obtained by both the techniques are in good agreement, while the surface studies support the conclusions drawn from the weight loss method. KEY WORDS: Corrosion; Mild steel; Acetic acid
1. Introduction It is well known that mild steel is one of the best preferred materials for industry due to its easy availability and excellent physical properties, but its use is restricted in acidic environments because of the susceptibility towards corrosion. It is cheaper than wrought iron and stronger and more workable than cast iron. On the other hand, from a commercial point of view, acetic acid is by far the most important organic acid among the lower carbon acids in the aliphatic series. It is applied in the manufacture of drugs and pharmaceuticals, rayon and plastics, rubber and silk industries. It is reasonable to expect mild steel to be used as reaction vessels or storage tanks by industries where the acetic acid is manufactured or used as a reactant. The effect of acetic acid on the kinetics of mild steel corrosion is a complex and serious problem and requires thorough investigation. It may be worthwhile to examine how carboxylic acids and their † Corresponding author. Ph.D.; Tel.: +91 98 68625466; E-mail address: sk− [email protected] (S.K. Singh).
derivatives are often used in conjunction with the organic corrosion inhibitors to enhance the efficiency of corrosion prevention and control practices[1] . In case of acetic acid, the acetate helps to passivate the corroding mild steel electrode. Aliphatic acids are adsorbed through their carboxylic groups and the inhibition is independent of the chain length[2] . Amino acid adsorption occurs through both protonated–COOH and NH2 groups[3] . Most organic inhibitors adsorb on the metal surface by displacing water molecules on the surface, forming a compact film[4–6] . It has been observed that short chain length carboxylates are not quite effective in inhibition of mild steel corrosion in neutral aqueous solution because they fail to block completely the surface sites for anodic dissolution[7] . A mixture of carboxylic acid and tertiary amine has been found to provide better inhibition for steel than either of the inhibitors taken individually[8] . Nitrogen is considered as the active centre of the amine. It is able to exchange electrons liberated by the metal during the first step of anodic dissolution. The physisorbed species react with the initial corrosion products to form a complex precipitate on the steel sur-
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269 [9,10]
face, which decreases the corrosion rate . Increase in the thickness of the layer is expected due to the synergistic effect of carboxylic acid and tertiary amine[10,11] . The behaviour of iron in neutral and weakly acidic solutions has been treated in relatively few publications. The corrosion behaviour of AISI 304L and 316L stainless steels, by Otero et al.[12] and 430 stainless steel in formic-acetic acid solutions have been studied by Sekine et al[13,14] . Abdel Aal et al.[15,16] studied the anodic behaviour of mild steel in deaerated carboxylic acid solutions and corrosion behaviour of steel in acetic, oxalic and citric acids. The influence of water content on corrosion of metals in monocarboxylic acids has been observed by Constantinescu and Heitz[17] . Performance of duplex stainless steels in acetic acid has been studied by Kangas and Newman[18] and a study on corrosion by lower aliphatic organic acids has been made by Teeple[19] for ferrous and non-ferrous alloys. The usual influence of hydrogen ions has been shown to apply to the dissolution process of iron in the solution of sodium salt of acetic acid by Nord and Bech-Nielsen[20] . Common corrosion problems encountered by various construction materials in acetic acid have been discussed in a report[21] . The effect of acetic acid on the corrosion of carbon steel for 500×10−6 –5000×10−6 concentration has been reported recently by Okafor and Nesic[22] . Corrosion behavior of 316L stainless steel in acetic acid solution over the concentration range 70%–90% has been studied by Turnbull et al[23] . Recently the corrosion behaviour of duplex stainless steels in organic acid aqueous solutions has been reported by Invernizzi et al.[24] and the effect of organic compounds on the electrochemical behaviour of steel in acidic media has been reviewed by Maksoud[25] . We have published the results of electrochemical behaviour of mild steel in glacial acetic acid with and without formic acid[26] . Marked dependence of corrosion behaviour on the concentration of the acetic acid and experimental temperature has been observed for AISI 316 stainless steel by Sekine et al.[27] and for mild steel by Singh and Gupta[28] . However, the investigation carried out by Singh and Gupta was for shorter exposure time and at selected small concentrations, and also did not report surface studies. In short, the kinetics of mild steel corrosion over the whole range of aqueous acetic acid solutions has not been represented adequately in literature. The aim of the present study was to investigate the corrosion behaviour of mild steel over a wide range of concentration and test immersion periods. The weight loss results have also been supplemented by surface study investigations. 2. Experimental In our experiments, analytical reagent (AR) grade glacial acetic acid and mild steel samples having composition of C, 0.23; Mn, 0.11; Si, 0.02; P, 0.02; S, 0.02;
265
Ni, 0.02; Cu, 0.01; Cr, 0.01; and remainder Fe (wt pct) were used. For weight loss experiments, coupons were cut in the form 3 cm×3 cm×0.1 cm. The surface of the specimen was polished using successive grades of emery paper, from 1/0 to 4/0, washed with soap solution and then by running tap water followed by double distilled water, finally degreased with acetone, dried in air and kept in a desiccator. Later, the samples were weighed and immersed in 300 mL of test solution in a glass beaker for a fixed interval of time. The samples were kept in the beaker in such a way that both the surfaces were in contact with the solution. After removing the specimens from the electrolyte, they were washed thoroughly, dried and reweighed. The surface morphology of the samples before and after performing the weight loss experiments was observed carefully and analyzed by a JEOL JSM-840A scanning electron microscope (Japan) at an operating voltage of 10 kV. For polarization measurements, the working electrodes were cut in rectangular form 3 cm×1 cm×0.1 cm. The exposed area was only 1 cm×1 cm and considering both sides the total exposed area was 2 cm2 . Remaining portion of the sample was covered with wax to avoid any possibility of electrolytic contact. A three-necked glass cell assembly with a 1 cm×1 cm counter electrode and a saturated calomel electrode (SCE) with a luggin probe containing saturated solution of KNO3 as a reference electrode was used. The open circuit potential (OCP) was recorded after it attained a constant value. The measurements were made at 25, 35 and 45◦ C and potentials were quoted with respect to the SCE. Potentiostatic polarization was achieved using a Wenking POS 73 potentiostat (Bank Elektronik, Germany), manually changing the potential stepwise at a rate of 10 mV/min. The temperature was kept constant with the help of an air thermostat with the variation of ±0.2◦ C in the experimental temperature. 3. Results and Discussion 3.1 Corrosion behaviour by weight loss method The corrosion rate of mild steel in different concentrations of acetic acid for exposure time 24 h and 168 h has been determined by weight loss method at 25, 35 and 45◦ C. The corresponding curves for specific conductivity and corrosion rate of mild steel as a function of concentration have been plotted in Fig. 1(a) and (b). It is observed that there is a rapid increase in the corrosion rate with increase in the concentration of acetic acid till it reaches a maximum at 25%. Any further increase in the concentration of acetic acid results in an exponential decrease in the corrosion rate of mild steel at each temperature. Similar results were found by Sekine et al.[14] for the stainless steel-acetic acid system. The marked decrease in corrosion rate at higher concentrations of acetic acid is assumed to be a consequence of the increase in the viscosity of
266
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
Fig. 2 Variation of corrosion rate of mild steel in 20% acetic acid with exposure time at different temperatures
Fig. 1 Variation of corrosion rate and conductance with concentration at different temperatures
the solutions due to the formation of a dimer, resulting in a decrease in the mobility of the ions. Acetic acid is generally associated to form a dimer and polymer at higher concentrations[29] . Besides this, an increased concentration leads to an increase in electrostatic ion-ion interactions and decrease in the degree of acetic acid dissociation. Maximum value of corrosion rate of SS41[29] in 30% and of copper[30] in 20% acetic acid have been reported. The corrosion behaviour observed for 168 h exposure is nearly the same as for 24 h. From the nature of curves in Fig. 1(a) and (b), it is evident that as the temperature of the system is increased, the corrosion rates of mild steel increase at all the concentrations of acetic acid, irrespective of the time of exposure. The dependence of corrosion behaviour of mild steel on acid concentration and temperature in the present case may also be explained in terms of the change in specific conductivity. The concentration of acetic acid for the maximum corrosion rate of mild steel almost coincides with the maximum in specific conductance. The increase in corrosion rate with temperature may likewise, be attributed to the increase in the conductivity of acetic acid solution as the temperature is increased. The corrosion rates of mild steel in 20% acetic acid solution have been determined by the weight loss method for 6, 12, 24, 48, 72, 96, 120, 144 and 168 h exposure time at 25, 35 and 45◦ C. The curves representing these values are shown in Fig. 2. The variation of corrosion rate of mild steel with exposure time at
20% acetic acid is found to be temperature dependence. It can be seen that initially with increasing time of exposure from 6 to 24 h, the corrosion rate of mild steel increases irrespective of temperature. However, on further increase of exposure time, the corrosion rates at 35 and 45◦ C exhibit different variation than that at 25◦ C. The variation in corrosion rate with the time of exposure between 6 and 24 h may be due to the increase in conductance of the solution as a result of continuous addition of Fe2+ caused by the corrosion of mild steel. The extent to which corrosion is promoted depends on the concentration of Fe2+ ions[31] . The decrease in corrosion rate after 24 h may be either due to a decrease in solubility of the electrolyte as it slowly becomes saturated with the corrosion product or the formation of some surface film which retards the corrosion of mild steel. The formation of a surface film is unlikely, as evident from the significant dissolution of the samples and also from the polarization data mentioned in the following section. The decrease in corrosion rate must have occurred due to precipitation of the corrosion product. This hypothesis is supported by the fact that at higher temperatures the corrosion rate starts increasing at a lower immersion period. Due to the enhanced rate at higher temperatures, the saturation limit would have been attained earlier. This behaviour is further supported by the results of surface study and has been discussed at 25◦ C as representative. As corrosion progresses in acetic acid solution, mild steel gradually starts dissolving in the acid. After 6 h of the immersion, the sample loses its metallic lustre and the polished streak is not seen on the surface. A similar phenomenon is observed after 12 h with more unevenness on the surface. After 24 h of exposure, the test sample is seen to be uniformly covered with a very thin non-sticky layer of dark gray corrosion product. On further increasing the immersion period, the amount of deposited material on the surface increases. Thus, the increase in corrosion rate
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
267
Fig. 3 Scanning electron microphotographs of mild steel specimens exposed to acetic acid solution (after weight loss experiment): (a) uncorroded polished samples, (b) 20% acetic acid for immersion period 24 h, (c) 20% acetic acid for immersion period 96 h, (d) 20% acetic acid for immersion period 168 h
observed by weight loss tests from 6 to 24 h is usually due to the increase of the exposed area because the surface becomes rough, leading to enhancement of the total surface area. Moreover, the number of Fe2+ ions increases in the solution due to continuous dissolution of material. Further, as the time of exposure increases beyond 24 h, the overall weight loss increases with time but the net corrosion rate shows a decreasing tendency up to 144 h. This decrease is initially fast but slows down with increasing time of exposure. This can be explained by the fact that the material deposited on the surface acts as a barrier leaving less active area on the surface. Corrosion process does not stop completely but progresses at a slow rate suggesting that there is formation of a nonadherent and non-compact film, which is unable to protect the material completely from further corrosion but only retards the process up to some extent. After a certain amount of deposition, protection is very slightly affected as it becomes controlled almost exclusively by porosity. It appears that after 120 h, the porosity attains a value that is practically unaffected by the amount of corrosion product presented on the surface. On further increasing exposure time after 144 h, the corrosion rate starts increasing again. It may be assumed that the deposited layer starts breaking beyond 144 h due to its porous and bulky nature. The surface morphology of the specimens was analyzed by scanning electron microscopy (SEM) after removing the corrosion product. The microphotographs are given in Fig. 3(a)–(d). Figure 3(a) represents the uncorroded, polished sample. The entire surface is smooth and homogeneous. Figure 3(b) shows the corroded sample exposed for 24 h, which corresponds to
the maximum corrosion rate. Figure 3(c) represents the corroded sample exposed for 96 h. This is the intermediate value of exposure time at which a decreasing tendency in corrosion rate is observed. Figure 3(d) indicates the corroded sample after 168 h of exposure, which is the maximum time of exposure. The SEM investigation results indicate a uniform attack by acetic acid. SEM analysis shows a very limited attack after 24 h (Fig. 3(b)). The corroded portion is much smaller than the total surface. With increase of the immersion period, the extent of corrosion is increased, which can also be seen from weight loss tests. After 96 h of the test, the surface appears uncorroded in several zones (Fig. 3(c)). This behaviour could be explained by assuming a layer of corrosion product acting as a barrier against liquid metal attack. The corrosion attack becomes more prominent and vigorous after 168 h (Fig. 3(d)) due to larger availability of the exposed area for attack. 3.2 Electrochemical behaviour of mild steel in acetic acid solution The potentiostatic anodic and cathodic polarization behaviour of mild steel in different concentrations of acetic acid, i.e. 5, 10, 20, 25, 30, 35, 40, 60, 80 and 90 percent has been studied at 25, 35 and 45◦ C, respectively. Figure 4 illustrates this behaviour of mild steel in different concentrations of acetic acid at 25◦ C. The figure shows active corrosion behavior of mild steel under all experimental conditions. Active nature of anodic curves for mild steel in 20% acetic acid solution has been shown by other investigators in the presence and absence of inhibitors[32] . Well-defined Tafel regions, followed by distinct limiting current densities are observed at all concentrations of the acid at all
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
268
Table 1 Electrochemical parameters of mild steel in different concentrations of acetic acid at 25, 35 and 45◦ C Concentration of acetic acid/% At 25◦ C 5 10 20 25 30 35 40 60 80 90 At 35◦ C 5 10 20 25 30 35 40 60 80 90 At 45◦ C 5 10 20 25 30 35 40 60 80 90
icorr /(μA/cm2 )
Ecorr /mV
iL /(mA/cm2 )
Anodic Tafel slope (βa )/(mV/decade)
Cathodic Tafel slope (βc )/(mV/decade)
Corrosion rate /mpy
72.44 95.50 151.36 151.36 125.89 120.23 104.71 66.07 19.95 3.55
−630 −667 −597 −620 −600 −600 −600 −545 −490 −435
5.01 6.67 7.58 7.24 6.02 4.17 3.31 1.58 0.25 0.036
160 172 188 180 192 192 204 200 214 285
200 204 227 220 226 208 250 219 196 250
33.15 43.70 69.26 69.26 57.61 55.01 47.92 30.23 8.33 1.62
112.20 173.78 260.02 260.02 199.53 181.97 165.96 107.15 36.31 10.0
−610 −655 −600 −615 −575 −580 −585 −575 −480 −460
5.49 7.94 13.18 11.79 7.58 4.36 3.63 2.19 0.46 0.095
150 185 217 250 192 192 200 208 250 250
238 155 273 250 250 250 250 211 289 389
51.34 79.52 118.98 118.98 91.30 83.27 75.94 49.03 16.61 4.58
190.55 257.04 389.19 389.19 354.81 346.74 275.42 144.54 52.48 14.12
−636 −625 −625 −600 −595 −600 −575 −535 −490 −410
6.61 8.71 15.84 12.58 10.47 5.25 4.36 2.63 1.99 0.46
166 200 230 293 250 278 285 250 275 239
500 294 278 429 286 375 321 346 500 425
87.19 117.62 178.09 178.09 162.36 158.67 126.03 66.14 24.02 6.46
Fig. 4 Anodic and cathodic polarization curves of mild steel in different concentrations of acetic acid at 25◦ C
the temperatures. The potential range over which the Tafel law followed does not depend much on the concentration of the electrolyte. This result is contrary to that obtained by Singh and Gupta[28] , whose observation for mild steel is not active but passive and transpassive behaviour as well. The values of corrosion current density (icorr ), corrosion potential (Ecorr ), limiting current density (iL ), the Tafel slope of anodic and cathodic curves (βa ) and (βc ) and corrosion rate calculated from the polariza-
tion curves at different concentrations of acetic acid at 25, 35 and 45◦ C, respectively, have been recorded in Table 1. By comparing these data with those of weight loss method, it may be inferred that the corrosion behaviour observed by these two methods is similar. However, at higher concentrations, viz. 80% and 90%, discrepancy between these two methods is observed. At the concentration of 90%, the electrochemical data exhibits tremendous decrease in the corrosion rate as compared with that obtained by weight loss method. This occurs due to the very high resistance of the solution at higher concentrations. Since acetic acid is a weak acid, it is feebly dissociated at higher concentrations. Thus, decrease in dissociation and increase in viscosity reduce its conductance. From Fig. 4, an initial regular shift in the cathodic polarization curves towards higher current density with increase in the concentration of the acid is observed. The curves become steeper as the concentration is increased. When the concentration is increased from 5% to 20%, the shift towards higher current density region is maintained over the whole range of potential. In the range from 20% to 40% acid concentration, the curves either overlap at some regions or even cross one another at others without showing any regular pattern.
S.K. Singh et al.: J. Mater. Sci. Technol., 2010, 26(3), 264–269
269
REFERENCES
Fig. 5 Plot of logicorr vs 1/T for various concentration of acetic acid
On the basis of above observations, the cathodic polarization behaviour of mild steel can be considered to be a hydrogen evolution reaction. With increase in the acid concentration, the concentration of undissociated acetic acid molecules increases, thereby decreasing free hydrogen ion concentration. The increase in cathodic reaction rate may be explained by the increase in the molecular concentration of acetic acid. The increase in corrosion rate with temperature is due to an increase in the extent of adsorption, assuming that the acetic acid is chemisorbed on the surface of the cathode. Ultimately, this leads to an increase in the rate of hydrogen evolution reaction. The plot of logicorr derived from the extrapolation of anodic and cathodic polarization curves at different concentrations of acetic acid against 1/T (T -absolute temperature), have been shown in Fig. 5. A linear relationship is observed indicating that these parameters are under activation control. 4. Conclusions (1) The corrosion behaviour of mild steel has a marked dependence on concentration, temperature and exposure time. A correlation between the corrosion rate and the state of the mild steel surface is observed for various immersion periods. 20%–30% acetic acid is more corrosive towards mild steel. (2) The anodic polarization curves of mild steel in acetic acid solution show active behaviour over the whole range of applied potential at each concentration and temperature. At higher concentration of acid (90%), the anodic current decreases due to very large internal resistance offered by the solution. (3) The cathodic reaction results in the evolution of hydrogen. The values of logicorr increase linearly with the decrease of 1/T . Acknowledgement The authors would like to gratefully acknowledge the many helpful suggestions provided by Prof. M.M. Singh, Department of Applied Chemistry, I.T., B.H.U., Vatanasi (India) in his discussion with the authors.
[1 ] R.D. Marco, Z.T. Jiang, D. John, M. Sercombe and B. Kinsella: Electrochim. Acta, 2007, 52, 3746. [2 ] M.N. Moussa, M.M. El-Tagoury, A.A. Radi and S.M. Hassan: Anti-Corros. Methods Mater., 1990, 37, 4. [3 ] R.M. Saleh and A.M. Shamseld: Corrosion Sci., 1972, 12, 689. [4 ] M. Bouklah, A. Ouassini, B. Hammouti and A. El Idrissi: Appl. Surf. Sci., 2006, 252, 2178. [5 ] S. Muralidharan, K.L.N. Phani, S. Pitchumani, S. Ravichandran and S.V.K. Dyer: J. Electrochem. Soc., 1995, 142, 1478. [6 ] P. Kern and D. Landolt: Electrochim. Acta, 2001, 47, 589. [7 ] U. Rammelt, S. Kohler and G. Reinhard: Electrochim. Acta, 2008, 53, 6968. [8 ] P. Bommersbach, C. Alemany-Dumont, J.P. Millet and B. Normand: Electrochim. Acta, 2005, 51, 1076. [9 ] N. Tsuji and K. Nozawa: Corrosion Sci., 2000, 42, 1523. [10] T. Suzuki, H. Nishihara and K. Aramaki: Corrosion Sci., 1996, 38, 1223. [11] M. Duprat, F. Dabosi, F. Moran and S. Rocher: Corrosion, 1981, 37, 262. [12] E. Otero, A. Pardo, M.V. Utrilla, F.J. Perez and C. Merino: Corrosion Sci., 1997, 39, 453. [13] I. Sekine, S. Hatakeyama and Y. Nakazawa: Corrosion Sci., 1987, 27, 275. [14] I. Sekine, S. Hatakeyama and Y. Nakazawa: Electrochim. Acta, 1987, 32, 915. [15] M.S.Abdel Aal and M.H. Wahdan: Brit. Corros. J., 1981, 16, 205. [16] M.S.Abdel Aal, M.H. Wahdan and G.K. Gomma: Mater. Chem. Phys., 1995, 39, 290. [17] E. Constantinescu and E. Heitz: Corrosion Sci., 1976, 16, 857. [18] P. Kangas and M. Newman: Anti-Corros. Methods Mater., 1998, 45, 233. [19] H.O. Teeple: Corrosion, 1952, 8, 14. [20] H. Nord and G. Bech-Nielsen: Electrochim. Acta, 1971, 16, 849. [21] H.O. Teeple: Corrosion, 1957, 13, 79. [22] P.C. Okafor and S. Nesic: Chem. Eng. Commun., 2007, 194, 141. [23] A. Turnbull, M. Ryan, A. Willetts and S. Zhou: Corrosion Sci., 2003, 45, 1051. [24] A.J. Invernizzi, E. Sivieri and S.P. Trasatti: Mater. Sci. Eng. A, 2008, 485, 234. [25] S.A. Abd El Maksoud: Int. J. Electrochem. Sci., 2008, 3, 528. [26] S.K. Singh, A.K. Mukherjee and M.M. Singh: IJCT, 2008, 15, 68. [27] I. Sekine, A. Masuko and K. Senoo: Corrosion, 1987, 43, 553. [28] M.M. Singh and A. Gupta: Corrosion, 2000, 56, 371. [29] I. Sekine and K. Senoo: Corrosion Sci., 1984, 24, 439. [30] V.B. Singh and R.N. Singh: Corrosion Sci., 1995, 37, 1399. [31] O. Krim, A. Elidrissi, B. Hammouti, A. Ouslim and M. Benkaddour: Chem. Eng. Commun., 2009, 196, 1536. [32] M.A. Quraishi and H.K. Sharma: J. Appl. Electrochem., 2005, 35, 83.